At Harvard Business School, there’s a case study on what makes one of the world’s most profitable law firms tick. They’ll have to rewrite it.

M&A king Wachtell Lipton Rosen & Katz’s move to London is hugely symbolic. It is the news most people in the market thought they would never see.

This is the firm that’s home to the legendary Marty Lipton, inventor of the poison pill defence. It is the most avowedly anti-internationalist firm in the US. It is the US firm that out-Slaughter-and-Mays Slaughter and May.

Well, they do say: never say never.

Wachtell has always been an anomaly. For a start, it’s the only firm I’ve ever come across that flags up its leverage on the home page of its website. It’s a minor point, but Wachtell sees the fact that its ratio of partners to associates is “far above” that of its major competitors as one of its key differentiators.

Also, don’t waste time looking for Wachtell acting for anyone other than the principal on deals. That’s where the cash is, and that’s where Wachtell can usually be found. It’s another way the firm maintains its ‘unique’ aura.

But, critically, Wachtell has rarely even considered expanding beyond its New York base. I say rarely because, as The Lawyer reported seven years ago, there is a precedent for its current London move.

In 2000 Wachtell flirted with the idea of launching a dotcom-related outpost in Silicon Valley. Don’t bother looking for it, by the way: it never opened.

But Wachtell’s lack of additional domestic or overseas offices has never been a barrier to it winning a place at the table on the world’s largest deals, wherever they might be.

Wachtell, without any global network, does damn well, nationally and internationally. Last year its average profit per partner stood at more than £2m.

Which makes today’s news all the more startling.

The year began with New York Mayor Michael Bloomberg warning the city he loves that it was in danger of losing its position as the world’s pre-eminent financial centre to London.

The year is drawing to a close with signs that the world’s pre-eminent deal-doing law firm is prepared to set up shop in the City.

What more proof do you need that the transatlantic balance of power is shifting?

To read Byrne in the USA from this week’s edition of TheLawyer magazine click here.

To read last week’s Byrne in the USA blogs from click here.